Cassop Pond, and Cassop Vale, looking towards Durham, April 2015.
One of my earliest posts described the algae that I found in Cassop Pond, which lies at the foot of the Permian escarpment close to my house in County Durham (see “Cassop”). I returned there a couple of days ago to grab a couple more samples and see what had changed since my last visit. The first of these samples was a handful of submerged plant stems, which I crammed into a sample bottle and shook to dislodge the algae. I pipetted a couple of drops of the brown suspension that this produced onto a microscope slide and put it under my microscope. Prominent amongst the diatoms that I could see in this sample was a nice colony of Gomphonema truncatum var. capitatum and also a cell of the large diatom Cymbella lanceolata, displaying its characteristic lobed chloroplast. I also saw a few cells of another diatom, Epithemia adnata. This diatom is relatively uncommon in the UK, but it gives us some interesting insights into the ecology of Cassop Pond.
Diatoms from Cassop Pond, April 2015. a. Gomphonema truncatum var. capitatum; b. Cymbella lanceolata; c. Epithemia adnata, valve view; d. Epithemia adnata, girdle view of two recently divided cells. Scale bar: 25 micrometres (= 1/40th of a millimetre).
Cells of the genus Epithemia contain small cyanobacteria-type cells that are capable of nitrogen-fixation (the reality is a little more complicated – see the reference by Prechtl et al. below for more information). This means that it can thrive in situations where nitrogen is relatively scarce compared to other nutrients. I came across it in a small stream in Northumberland, downstream of a forestry plantation. I was interested in the stream because the plantation was being fertilised with phosphorus at the time and I wanted to see what this would do to the stream. Every time it rained (which was quite often in the Northumberland hills), some of the phosphorus was washed into the stream and, gradually, over a period of about a month after the fertilisation, the proportion of Epithemia in my samples increased from undetectable at the start to over forty percent of all the diatoms a couple of months later. What I suspect was happening was that most of the algae in this remote stream could not use this extra phosphorus because nitrogen was naturally very scarce. However, Epithemia is one of a very small number of diatoms that can overcome nitrogen limitation and so was able to thrive. Finding it in Cassop Pond is, therefore, a clue that this pond is, periodically at least, limited by nitrogen rather than by other nutrients.
The other photograph I’ve included in this post was taken by Chris Carter and shows Epithemia growing on the surface of Chara virgata. Epithemia is a genus that does often seem to be associated with plants, although I have also seen it growing on rocks. Chris’ photograph also shows the lobed chloroplast very clearly.
Epithemia sp. growing on a stem of Chara virgata. Photograph by Chris Carter.
DeYoe, H.R., Lowe, R.L. & Marks, J.C. (1992). The effect of nitrogen and phosphorus on the endosymbiont load of Rhopalodia gibba and Epithemia turgida (Bacillariophyceae). Journal of Phycology 23: 773-777.
Kelly, M.G. (2003). Short term dynamics of diatoms in an upland stream and implications for monitoring eutrophication. Environmental Pollution 125: 117-122.
Prechtl, J., Kneip, C., Lockhart, P., Wenderoth, K. & Maier, U.G. (2004). Intracellular spheroid bodies of Rhopalodia gibba have nitrogen-fixing apparatus of cyanobacterial origin. Molecular Biology and Evolution 21: 1477-1481.